The invention relates to an optical sensor system based on attenuated total reflection (ATR), and more particularly to a surface plasmon resonance based optical sensor system and a method for sensing.
Surface plasmon resonance (SPR) based sensors are commercially available for use in research and development. For example, SPR sensors are available from the BIACORE® instrument line from GE Healthcare, Uppsala, Sweden. These commercially available instruments use a sensor glass chip covered with a thin gold film carrying an immobilized chemical sensor layer, and an integrated fluid cartridge for passing sample fluid and other fluids over the sensor chip. A wedge shaped beam of light is coupled to the sensor chip via a prism and a reusable optical interface, such that an angular range of incident light is reflected internally along a line at the glass/gold film interface thus creating a total internal reflection (TIR)-evanescent field at the glass/gold interface. At a unique narrow range of angles for a particular wavelength, this TIR-evanescent field transfers energy from incident light rays through the gold film and creates a surface plasmon wave resonance at the gold film/sensor layer interface. The surface plasmon wave generates an enhanced evanescent electric field that has a characteristic penetration depth into the sample side of the gold surface whereby the refractive index of the sample determines the SPR-angle. A photodetector 2D-array detects the reflected light intensity distribution versus angle of incidence for a row of sensor spots along the illuminated line to produce simultaneously a SPR spectrum for each sensor spot. Upon imaging these multi SPR spectra onto the photodetector the image has bright and dark bands. The sensor measures the angular location of a dark band on the detector surface generated by the resonant coupling of the reflected light and going into the gold film as surface plasmon energy. The angular location of the surface plasmon resonance depends on the refractive index of the sample being penetrated by the SPR evanescent field. The amount of reflected energy will also depend on the degree of absorption of evanescent field energy, as is the case for a sample having a complex valued refractive index at the chosen wavelength.
High sensitivity and high resolution of SPR-spectroscopy is desirable, particularly in the case of kinetic studies. Also in the field of high-throughput biomolecular screening, high sensitivity is desirable for SPR-spectroscopic and other ATR-spectroscopic methods.
The sensitivity or resolution in the detectable change of the angle (or wavelength) at the dip or peak (or in some cases, dips or peaks), or centroid (center-of-mass) of the SPR reflection curve is mainly limited by the degree of constancy, drift and noise of the background light intensity of the total internal reflection curve (TIR-curve). Ideally, the TIR-curve is constant with respect to the angle of incidence. In practice, however, due to variation of reflectance with the angle of incidence, and due to the radiation distribution from the light source, the emitted light beam cross intensity profile, and as a consequence also the TIR-curve is generally a Gaussian type curve with at least one maximum. The reflectance may vary due to several reasons, such as, reflection losses in the coupling of light between prism (or grating) and the plasmon supporting metal. A constant background intensity pattern can be normalized by a suitable software algorithm. However, a changed background image and/or too large correction will introduce a “normalization error”. Irrespective of the type of algorithm used for calculation of the dip, peak, centroid (center-of-mass), etc. ATR-spectrum curvature characteristics, this “normalization error” must be minimized in a high resolution ATR-sensor instrument. Using normalization over a large variation in intensity across the detector array leads to a decreased signal to noise ratio in the lower intensity areas on the edge of the detector array. Generally, the differences between detected Gaussian-type light source intensity distributions and mathematically created, and measured normalized intensity distributions are too large.
For high sensitivity or high resolution of ATR-sensor devices, and in particular SPR-sensors, it is desirable that some area of interest on the sensor surface be illuminated as uniformly as possible, providing a TIR-curve with flat-top intensity profile.
A conventional surface plasmon resonance (SPR) measurement system typically comprises one or more light emitting diodes (LEDs) that at TIR illuminate the surface plasmon sensor device. LEDs have a coherence length that is long enough to enable an SPR measurement system to detect small shifts in SPR resonances. The ability to detect small shifts in SPR resonances enables the system to be highly accurate and highly sensitive or to otherwise improve the resolution.
Typically, light from an LED is Lambertian (diffuse light emitted into a hemisphere with a cosine drop off in intensity) and has low optical power. These properties of the LED can reduce the amount of light that is incident on an ATR-sensor device and decrease the signal-to-noise (SNR) ratio, which can correspondingly reduce the accuracy and sensitivity or resolution of the SPR measurement system. The required high optical power incident on the ATR-sensor may be delivered using edge-emitting light sources, like edge-emitting diodes, superluminescent diodes, and laser diodes, due to their highly directional beam of high intensity in a narrow radiation angle.
High power solid-state light sources, like edge-emitting diodes, superluminescent diodes, and laser diodes, produce a highly directional beam of light. Also, when these light sources are coupled to optical waveguides or fibers, the output optical beam from the waveguide or fiber is also highly directional. However, most of the high power sources have an extremely inhomogeneous light intensity. Specifically, the light intensity of a high power beam follows a Gaussian distribution. The non-uniform or Gaussian type intensity distribution of the beam incident onto the detector limits the sensitivity and resolution of the ATR-spectrometer when these light sources are either directly used in SPR-sensing, or are followed by a focusing optics.
The optical power inhomogeneity limits ATR-sensors, because the resolution is too low for biological interaction. For example, simultaneous high resolution and accurate detection of multi spot arrays is necessary in the field of high-throughput screening for pharmaceutical development. Therefore, there is a need for a high power (directional) beam that has a uniform optical intensity.